Optical fiber connectors are a critical part of essentially all optical fiber communication systems. For instance, such connectors are used to join segments of fiber into longer lengths, to connect fiber to active devices, such as radiation sources, detectors and repeaters, and to connect fiber to passive devices, such as switches, multiplexers, and attenuators. The principal function of an optical fiber connector is to hold the fiber end such that the fiber's core is axially aligned with an optical pathway of the mating structure. This way, light from the fiber is optically coupled to the optical pathway.
A light wave propagating in a standard single mode fiber has a fundamental propagation mode (TEM00), which is a combination of two, orthogonally-polarized modes travelling at the same velocity with a common refractive index for both modes due to material symmetry of the glass fiber. Theoretically, these two modes are functionally identical, however, any deviation from a straight non-stressed fiber having perfect geometry will cause interaction and cross coupling between the two propagating modes. This cross coupling occurs when the fiber is exposed to thermal or physical influences which introduce local refractive index changes resulting in interchange of energy between the propagating modes. Considering that the speed of light in a PM fiber is inversely proportional with the magnitude of the refractive index (n), even small fiber disturbances can cause polarization variations. Thus, for any practical setup outside of a controlled laboratory environment, use of a regular single mode fiber will result in an uncontrolled energy cross coupling between the modes, an effect which is called birefringence. The refractive indices for the two modes will vary depending on the level of the induced stress causing a random mode propagation with different phase velocities. This modal energy transfer—crosstalk—results in a random state of polarization exiting the fiber and a pulse broadening called Polarization Mode Dispersion (PMD).
For certain applications, it is advantageous to use fibers designed with built-in birefringence which have the ability to maintain the linear polarization and preserve it even if the fiber is exposed to (limited) mechanical stress or external thermal influences. These are called polarization maintaining (PM) fibers. PM fibers maintain a stable polarization state in single mode optical transmission. The main applications for PM optical fibers are in sensors, interferometers and optical gyroscopes. They are also used frequently in telecommunication as a connection between a laser and an E/O modulator which requires an optical polarized input. Short fiber lengths are preferred due to the relative high cost of PM cables but also because they tend to have higher attenuation than regular single mode fibers.
There are three (3) main PM fiber types—namely, the ‘Panda’ fiber, the ‘butterfly’ or ‘bowtie’ fiber and the oval PM fiber. Each type has high expansion glass stress members arranged symmetrically in relationship to the fiber core. During cooling at the fiber drawing process, these stress members shrink slightly more than the surrounding glass and will cause a permanent tension on the core material. This directionally controlled tension induces birefringence, which means that two different indices of refraction are created in the fiber in directions substantially perpendicular to each other. The more birefringence that is generated, the more stress is applied to the core, and the larger the velocity difference is between the two modes, and the more difficult it is to induce cross-coupling via external stress applications. The axis of the applied stress—namely the slow axis—results in a higher refractive index than that created perpendicular to the stress application. Therefore, if linear polarized light is launched into the fiber in the same plane as the slow axis it will travel at a lower velocity (n high) than if it had been launched into the perpendicular plane of the fast axis (n low). Because of the difference in propagation velocity, the energy cross-coupling between the modes is prevented and the polarization state of the light wave is preserved.
The polarization maintaining ability of a PM fiber can be established by measuring the polarization extinction ratio (PER). This property is defined as 10 times the logarithm of the maximum intensity along the direction of the polarization divided by the minimum intensity of the component in the orthogonal direction. The unit of the PER calculation is expressed in dB.
  PER  =      10    ·          log      ⁡              (                              P            ⁢                                                  ⁢            max                                P            ⁢                                                  ⁢            min                          )            PM fibers with high birefringence can sustain more than a 30 dB polarization extinction ratio. In other words, the intensity of light in the first axis has approximately 1,000 times more energy than that of the perpendicular axis having minimum intensity.
Interconnection of PM fibers requires a connector technology that is able to align not only the fiber cores to realize low mating loss, but also the polarization axes of the fibers to achieve a precise angular alignment of the mated polarization planes. This is necessary to avoid cross-coupling and a degradation of the extinction ratio. Specifically, a sensitivity analysis of the PER dependence can be done geometrically by use of the misalignment angle θ between the two mated slow axis.PER=10 log(tan2 θ)Here θ is the misalignment angle between two polarization angles of the mated fibers. The PER function of the angular offset is shown graphical in FIG. 2. This graph can be used as a quick reference to determine the maximum misalignment angle allowed for a certain PER level. When the angle is known, calculations can be made to establish the mechanical tolerances needed to achieve the targeted design objective. For example, to sustain a PER of 30 dB or better, across the optical interface, the mechanical tolerance between the fiber polarization axis must be less than 1.8 degrees.
Therefore, a need exists for an optical connector that precisely aligns the PM fibers to reduce angular offset such that the extinction ratio is minimized. The present invention fulfills this need, among others.